This invention generally relates to a process for causing the development of images in electrostatographic systems, and more specifically, the present invention is directed to an improved process for accomplishing the development of electrostatic latent images with a synthetic developer composition. In one embodiment the process of the present invention is accomplished in a development apparatus wherein there is provided an agitated development zone encompassed by an imaging member and a transporting member, which in a preferred embodiment is comprised of rotating magnets contained in a stationary shell. Synthetic developer particles contained in the development zone are caused to desirably agitate by the relative movement of the imaging member and rotating magnets. The process of the present invention allows the continual development of high quality images, including the efficient and effective development of solid areas.
The development of images by electrostatic means is well known. Thus, for example, in these systems, toner particles are applied to electrostatic latent images by various methods including cascade development, magnetic brush development, powder cloud development, and touchdown development. Cascade development and powder cloud development methods were found to be especially well suited for the development of line images common to business documents, however, images containing solid areas were not faithfully reproduced by these methods. Magnetic brush development systems, however, provide an improved method for producing both line images and solid area.
In magnetic brush development systems it is usually desirable to attempt to regulate the thickness of the developer composition, which is transported on a roller by moving the roller past a metering blade. The adjustment of the metering blade is important since in the development zone the flow of developer material is determined by a narrow restrictive opening situated between a transport roller and the imaging surface. Accordingly, in order to provide sufficient toner particles to the imaging surface, it is generally necessary to compress the developer bristles, thereby allowing toner particles adhering to the carrier particles near the ends of the bristle to be available for development. Any variation, or non-uniformity in the amount of developer metered onto the transport roller, or into the spacing between the transport roller and imaging member can result in undesirable developer flow, and non-uniform image development. Non-uniform development is usually minimized by carefully controlling developer runout on the transport roller, and on the imaging member, and by providing a means for side-to-side adjustment in the relative positions of the metering blade, development roller and imaging member.
Moderate solid area development with magnetic brush is usually achieved by transporting the developer composition on a roller at a speed that exceeds the process speed of the image bearing member. At high process speeds the development-transport roller speed is limited by centrifugal forces, which forces cause the developer material to be removed from the roller. Thus, in order to obtain moderate solid area development at high process speeds, the use of multiple development rolls is necessary for increased developability.
The developer materials presently used in magnetic brush development differ widely in their electrical conductivity, thus at one extreme in conductivity, such materials can be insulating, in that a low electrical current is measured when a voltage is applied across the developer. Solid area development with insulating developer compositions is accomplished by metering a thin layer of developer onto a development roll, which is in close proximity to an image bearing member, the development roll functioning as an electrode, and thus increasing the electrostatic force acting on the toner particles. In these systems, the spacing between the image bearing member, and the development roller must be controlled to ensure proper developer flow, and uniform solid area development, the minimum average spacing generally being typically greater than 1.5 millimeters.
Insulating developer compositions can be rendered conductive by utilizing a magnetic carrier material which supports a high electric current flow in response to an applied potential. Generally, the conductivity of developer compositions depends on a number of factors including the conductivity properties of the magnetic carrier, the concentration of the toner particles, the magnetic field strength, the spacing between the image bearing member and the development roll, and developer degradation due to toner smearing on the carrier particles. Also, when insulative toner particles are permanently bonded to a conductive carrier, the conductivity decreases to a critical value below which solid area development becomes inadequate, however, within certain limits the process and material parameters can be adjusted somewhat to recover the decrease in solid area developability.
When using conductive developer materials in electrostatographic imaging systems, the development electrode member is maintained at a close effective distance from the image bearing member, and a high electrostatic force acts only on those toner particles which are adjacent to the image bearing member. Accordingly, since the electrostatic force for development in such systems is not strongly dependent on the developer layer thickness, the uniformity of solid area development is improved despite variations in the spacing between the image bearing member and the development roller member. More specifically, for example, in magnetic brush development systems utilizing conductive developer materials, solid area deposition is not limited by a layer of net-charged developer near the imaging member, since this charge is dissipated by conduction to a development roller. The solid area deposition is, however, limited by image field neutralization; provided there is sufficient toner available at the ends of the developer brush, which toner supply is limited to the ends or tips of the bristles, since toner cannot be extracted from the bulk of the developer mixture; wherein high developer conductivity collapses the electric field within the developer at any location, and confines it to a region between the latent image and the developer. For either insulative or conductive developer, solid area deposition is limited by toner supply at low toner concentrations, and the toner supply is limited to a layer of carrier material adjacent to the image bearing member, since the magnetic field stiffens the developer, and hinders developer mixing in the development zone.
In the above-described systems, undesirable degradation or deterioration of the developer particles results. This is generally caused by a variety of factors, including for example, the frequency and intensity of collisions between adjacent carrier particles contained in the developer composition, which collisions adversely affect the developer conductivity, and the triboelectric charging relationships between the toner particles and magnetic carrier particles. Thus, for example, a decrease in the triboelectric charge on the toner particles causes an increase in solid area development, and an increase in the amount of toner particles that are deposited in the background, or normally white areas of the image, accordingly, in order to maintain the original image quality in such situations, the triboelectric charge on the toner particles is increased, by reducing the concentration of such particles in the developer composition mixture. Also, when the toner charge, and toner concentration decreases, the developer material must be replaced in order to obtain images with acceptable solid areas decreased background.
While several improved types of toner and carrier materials, as well as processes have been developed for the purpose of developing images, difficulties continue to be encountered in the design of a simple, inexpensive, and reliable two-component development system which will provide a high solid area development rate, low background deposition, and long term stability. The present magnetic brush systems are inherently inefficient primarily since only a small fraction of the toner transported through the development zone is accessible for deposition onto the image bearing member. For insulative developer, the solid area deposition is limited by a layer of net-charged carrier particles produced by toner development onto a precharged imaging member. Since the developer entering the development zone has a neutral charge, deposition of charged toner onto the imaging member produces a layer of oppositely charged developer which opposes further toner deposition. Also, the net electrostatic force due to the charged image member, and the net-charged developer layer becomes zero for that toner between the developer and the electrostatic latent image of the imaging member, and a collapse in the electrostatic force, or the electric field acting on the charged toner, occurs even though the toner charge deposited on the photoreceptor does not neutralize the image charge. Image field neutralization can be approached, however, if there is a sufficiently high developer flow rate, and multiple development rollers. Image field neutralization results when the potential due to a layer of charged toner deposited on the imaging member is equal but opposite to the potential due to the charged imaging member. In the absence of a bias on the development roller, image neutralization produces a zero development electric field, and since the toner layer is of finite thickness, the charge density of the toner layer is less than the image charge density. Should the thickness of the charged toner layer be much less than the imaging member, image field neutralization occurs when the toner charge density neutralizes the image charge density.
There is disclosed in U.S. Pat. No. 4,394,429 and U.S. Pat. No. 4,368,970, an imaging process and apparatus containing a development means which is comprised of a tensioned deflected flexible imaging member, and a transporting means with a development zone being situated between the imaging means and the transporting means. The development zone contains therein electrically insulating toner particles and electrically insulating magnetic carrier particles. Movement of the flexible imaging member means and the transporting means in opposite directions at the different speeds causes the developer particles contained in the development zone to desirably agitate. In this process, however, a magnetic field is not present and furthermore a synthetic developer composition is not selected for use in the system.
Furthermore, there is described in U.S. Pat. No. 4,376,813, an improved reversal development method which involves forming a magnetic brush around an outer circumferential surface of a developing sleeve accommodating a magnet therein by the use of a developer composition comprised of high resistivity magnetic toner, and rubbing a surface of the electrostatic latent image with the magnetic brush. Additionally, U.S. Pat. No. 4,345,014, discloses a magnetic brush development method wherein there is selected a dual component development material which includes electrically insulating magnetizable particles as carrier substances, and electrically insulating non-magnetic particles as a toner composition. Accordingly in this patent, there is illustrated a developer composition which is comprised of carrier particles of for example magnetite, ferrite, or pure iron containing therein a bonding material, such as heat hardening resins including phenolic resins, reference Col. 3, beginning at line 60 of the U.S. Pat. No. 4,345,014.
Moreover, there is described in U.S. Pat. No. 4,344,694, a development apparatus wherein there is selected as developing component, toner particles containing a ferromagnetic material in powder form, or a mixture of toner and carrier particles which may contain iron particles or other ferromagnetic material, reference Col. 2, beginning at around line 40.
The art of xerography nevertheless continues to advance and there continues to be a need for improved processes and apparatuses for causing development of images in an efficient and economical manner. Additionally, there continues to be a need for improved processes wherein there is obtained images of high quality and excellent resolution. Furthermore, there continues to be a need for improved processes wherein there is selected as a developer compositions toner resin particles and carrier resin particles. Furthermore, there is a need for an improved development process wherein the carrier particles are within a size range so as to prevent bead carry out or consumption of the carrier particles during the electrophotographic process. By bead carry out in accordance with the process of the present invention is meant carrier particles sticking to the photoreceptor during development and being carried out of the development sub-system and consumed either on the output copy or in the cleaner assembly. Additionally, there continues to be a need for the provision of an improved process wherein background development in substantially eliminated and wherein the life of the developer composition is increased.